1. Field of the Invention
The present invention relates generally to the field of active semiconductor devices. More specifically, the present invention relates to novel semiconductor device structures useful in low voltage and high current density applications. More particularly, the present invention relates to active semiconductor devices referred to as normally off Field Effect Transistors (FET), which specifically include Junction Field Effect Transistors (JFET) as well as Metal Semiconductor Field Effect Transistors (MESFET).
2. Related Art
The increasing trend toward lower supply voltages for active semiconductor devices and Integrated Circuits (IC's) has accelerated the search for more efficient low voltage power sources. Conventional power supplies utilizing silicon diode rectifiers are unacceptable in low voltage applications due to the excessive voltage drop across the forward biased diode terminals. Power loss in the diodes becomes excessive when they are used as rectifiers in a direct current (dc) power supply designed for a terminal voltage as low as 3.0 volts.
Semiconductor diodes are combined with active devices to form circuits capable of producing low value dc supply voltages, but such circuits are generally not capable of handling the large currents frequently required. They usually exhibit a fairly large internal resistance and as such are very inefficient power sources. Furthermore, the number and complexity of steps required in the processing of this type of circuit as an IC also increases with the number of devices included.
Active semiconductor devices are used as switches in circuit arrangements producing dc power supply voltages, as for example in switched mode power supplies. Junction Field Effect Transistors (JFET) can be used as switches because they are easily switched between an on or conducting state and an off or non-conducting state. Most importantly, the current carriers in a JFET are all majority carriers which results in short switching times. However, when operated at lower voltages, JFETs exhibit an internal resistance in the on state that make them unsatisfactory and inefficient in applications requiring large currents.
In U.S. Pat. No. 4,523,111 entitled “Normally-Off Gate-Controlled Electric Circuit with Low On-Resistance”, Baliga disclosed a JFET serially connected to an Insulated Gate Field Effect Transistor (IGFET). The on resistance of this circuit is the sum of the JFET resistance and the IGFET resistance. As a result, the on resistance is too large and therefore unsatisfactory for low voltage operations requiring large currents.
In a similar invention disclosed in U.S. Pat. No. 4,645,957 entitled “Normally Off Semiconductor Device with Low On-Resistance and Circuit Analogue” by Baliga, a JFET is serially connected to a Bipolar Junction Transistor (BJT). The on resistance is the sum of the JFET and the BJT which is again too large for low voltage applications requiring large currents.
The previously cited U.S. patent application Ser. No. 09/430,500, “NOVEL JFET STRUCTURE AND MANUFACTURING METHOD FOR LOW ON RESISTANCE AND LOW VOLTAGE APPLICATIONS”, Ho-Yuan Yu, filed Dec. 2, 1999, discloses the basic structure for novel semiconductor devices useful for switching high level currents in ac circuit applications. These novel semiconductor devices have very low on resistance, and could be useful as switches in circuit arrangements producing dc power supply voltages, as for example in switched mode power supplies. Furthermore, the current carriers in these devices are all majority carriers which would result in short switching times. However, in dc circuit applications at voltage levels greater than approximately 0.4 volts, the normally off JFET disclosed will not easily switch between an on or conducting state and an off or non-conducting state. The normally off JFET wil not easily used as an amplifier under dc bias above 0.4 volts. Therefore a need of starter device to assist normally off JFET to be used as a switch or an amplifier under dc bias above 0.4 volt.